It is known that transistors can be employed as switches in electrical circuits, wherein the transistor may either be conducting or non-conducting. A transistor, for example, power MOSFETs, thus can be employed as a switch for coupling an electrical load to an electrical energy source.
Upon switching the transistor to its conducting state the electrical load is coupled to the energy source and a current will flow through the transistor to the load. Similarly when switching a transistor off, i.e., switching the transistor from its conducting to its non-conducting state, the electrical load, in particular when comprising an inductor, may discharge its stored energy causing a power peak in the transistor upon switching off. In conventional circuits a transistor simply may be designed to stand the current, i.e., the dimensions of the transistor are chosen such that under normal operating conditions the current will not overload the transistor.
For higher currents a plurality of transistors can be switched in parallel, such that each of the plurality of transistors carries only a part of the total current. Although the current in this case is distributed across the plurality of parallel transistors each of the plurality of transistors is designed for carrying only a portion of the total current. The problem of overload thus remains for each transistor.
However operating conditions may deviate from normal operations, for which the transistor has been designed, and a higher current may flow through a transistor. For example in case of a short on the load side of the transistor an unusual high current may flow causing a current density in the transistor exceeding the allowed range.
A current causing a current density exceeding the allowed range may destroy the transistor by heating the semiconductor structure, such that the transistor is locked in its conducting state and cannot be switched off by applying an appropriate gate voltage. In this case the transistor is destroyed.
Hence a transistor operated as a power switch should be protected from overload operating conditions. In conventional circuits transistors are protected by either increasing the resistance in the circuit, for example, by controlling the gate voltage to increase the resistance of the transistor and thus limiting the current, or by switching the transistor off, in case a sensed current exceeds a predefined maximum.
In conventional circuits the current limit for switching a transistor off is defined to be static for a particular load. Hence there is a need for an improved method and circuit providing a more flexible protection of a transistor.